Hinges on a conventional, articulated helicopter rotor hub permit flapping motion of the blades, pitching movement of the blades, and lead/lag movement of the blades, but add to the overall mass and weight of the helicopter hub. In addition, the hinges require lubrication and constant observation to detect wear and fatigue.
Hingeless helicopter rotor assemblies using composite materials, are being developed in response to the aforementioned limitations to provide light weight helicopter hub assemblies. A hingeless helicopter rotor assembly typically includes a rigid central hub member and radial flexible beams (flexbeams) rigidly attached by their root (inboard) ends thereto. The blades are rigidly attached at their root ends to the outboard ends of the flexbeams. A flexbeam may comprise at least one C-beam, I-beam, T-beam, X-beam, etc.
One flexbeam embodiment comprises two back-to-back C-beam members. The flexbeam is designed to bend in the vertical mode to accommodate blade flapping and in the horizontal mode to accommodate lead/lag motion of the blades. A rigid pitch shaft disposed between the two C-beam members transmits blade pitch-changing inputs from a control rod near the hub to the root end of the blade, and the flexbeams yield torsionally to permit the blade pitch changes. Thus, flexbeams must be resilient in three orthogonal axes (pitch, lead/lag and flap), but of course are longitudinally rigid to transmit blade centrifugal force to the central hub. Such an arrangement is the Sikorsky Elastic Gimbal Rotor which is disclosed in greater detail in Paper No. A-84-40-17-8000 presented at the 40th Annual Forum of the American Helicopter Society on May 16-18, 1984. This hingeless helicopter rotor with elastic gimbal hub is illustrated in U.S. Pat. No. 4,323,332.
The rotating blades in a helicopter hub assembly are known to go through a variety of predictable cyclic position changes due to external vibratory forces. For example, alternating position due to one per revolution air forces with forward flight as well as coriolis forces as a result of dynamic coupling of flapping and lead/lag motion act on each rotor blade throughout a revolution. Other possible vibratory forces include those associated with the rotor shaft motion when the helicopter is sitting on the ground and naturally rocking on its landing gear.
If a condition is encountered such that any single rotor mode (i.e., flap, lead/lag, pitch) is forced to vibrate at a frequency close to the natural frequency of that mode, resonance will take place with potentially catastrophic results. Without proper damping the amplitude of the oscillations could increase to destructive magnitudes. This is especially a problem for the lead/lag mode as both the flapping mode and the pitch mode are inherently adequately damped. Flapping motion is naturally damped by aero-dynamic forces and pitch motion is naturally damped through the control system. Lead/lag motion, however, has little or no natural damping.
It is known to include mechanical vibration dampers in conventional helicopter rotor assemblies to provide mechanical damping for the lead/lag mode and thereby avoid the potential problems should external vibratory forces approach the lead/lag made natural frequency. This is typically done by connecting hydraulic damping cylinders from the blade to the hub assembly.
In a hingeless helicopter hub assembly using flexbeams, as discussed above, it is not practical to attach a damping device directly between the hub and the blade because of the intervening flexbeam, which may be as long as 20% of the blade length. In addition, the amplitude of the lead/lag motion of a helicopter blade in a hingeless rotor assembly tends to be less than the amplitude in an articulated rotor system. Accordingly, the movement of the flexbeam is substantially less than that of the blade in the horizontal position since the amount of displacement is a function of radial (longitudinal) distance from the root end of the flexbeam. Although it would be possible to use a hydraulic damping means to damp the flexbeam lead/lag motion, conventional hydraulic dampers have the disadvantages of requiring high maintainance, having relatively low efficiencies, and being relatively large and heavy.